Plasma device with resonator circuit providing spark discharge and magnetic field

ABSTRACT

The present invention relates to a procedure and a device for forming a plasma. The plasma generated can be used e.g. to examine the concentrations of elements contained e.g. in different gases, such as flue gases. The spark discharge and magnetic field used to form and maintain the plasma are produced by means of the same capacitor-coil resonator circuit. The device of the invention allows a very stable and controlled plasma to be achieved.

BACKGROUND OF THE INVENTION

The present invention relates to a procedure and to a device for forminga plasma.

Elementary analyses of gas or aerosol samples are currently performed bysubjecting a sample gas flow to a high temperature using externalenergy. Generally the sample gas is mixed with a gas that easilytransforms into plasma, e.g. argon, helium or nitrogen, which may alsobe a component of the gas mixture under analysis. When the sample gasbecomes sufficiently hot, the electrons in the in atoms of the elementsbecome excited, and the wavelength of the light quantum or photonproduced when the electrons are de-excited is characteristic of eachelement and its electron ring. By examining the light quanta, it ispossible to determine the elements and their amounts contained in thesample.

As is known, the external energy can be produced using various systems.Previously known is an induction heater, which uses magnetic flux totransfer energy into the gas to be heated. A problem with the use ofmagnetic flux is how to "ignite" the gas, i.e. how to achieve asufficient degree or ionization to induce the plasma state of the gas. Asmall gas quantity cannot receive a sufficient amount of energy from themagnetic flux, and this leads to the need for large apparatus using ahigh volume of gas flow. On the other hand, if small amounts of gas areused, the magnetic field has to be generated using a very highfrequency, typically a frequency of several gigahertz. Conventionally,this problem is solved by using a spark between two electrodes to"ignite" the gas. The spark is created in the area where plasma is to bedeveloped and it is extinguished after a plasma flame has been set up.This is not an automatic system, because if the plasma decays inconsequence of an external disturbance, such as a power failure, gassupply failure or the like, it has to be ignited again with a spark.

Another prior-art method is to use only a high-voltage spark to producea plasma. In this case, a gas is ionized using an electric spark until abreakdown occurs and the gas is converted into plasma. However, thespark is not extinguished after a plasma has been generated, but thespark is used to transfer the energy required by the plasma to the gas.As the required high power is transferred by means of a spark, the sparkdischarge is very unstable and difficult to control, causing serious;disturbances in the analysis of sample gases.

SUMMARY OF THE INVENTION

The object of the percent invention is to eliminate the drawbacksmentioned above.

A specific object of the present invention is to produce a procedure anda device for forming a plasma which allow a stable and controlled plasmato be generated with flue gas samplets for the purpose of determiningthe percentages of elements present in the flue gas samples.

Another object of the present invention is to produce a plasma formingdevice that works on a continuous principle, i.e. when the plasmareverts into gas, the device acts automatically so that the gas is againconverted into plasma.

A further object of the present invention is to produce a procedure anda device which enable a plasma to be generated and maintained with apower demand significantly lower than in prior-art devices.

As for the features characteristic of the invention, reference is madeto the claims.

In the procedure of the invention for forming a plasma, a magnetic fieldis set up in a plasma forming space, a spark discharge is produced inthe plasma forming space and a gas flow is passed into the plasmaforming space against the magnetic field. Preferably the gas flow isapplied in a direction perpendicular to the magnetic field, permittingthe most effective transfer of electric energy from the magnetic fieldto the gas. According to the invention, plasma is generated in theplasma forming; space by means of the spark discharge and maintained bymeans of the magnetic field and spark discharge, in practice, however,the situation is such that when the power of the magnetic field issufficient, the spark has only a slight significance for the plasma.However, since the spark discharge exists continuously, the procedure ofthe invention is automatic,

As compared with prior art, the present invention has the advantage thatplasma is formed automatically both at the first "ignition" and duringoperation when the plasma has reverted back into gas due to adisturbance. Moreover, the arrangement of the invention allows asignificant reduction in the energy consumption. This is because in thedevice of the invention the energy can be applied accurately to theplasma forming region and used for the generation of plasma. Inaddition, the circuit used in the device of the invention has a goodefficiency.

Another advantage of the present invention as compared with prior art isthat no high voltage or high power needs to be used in the amplifierwhich feeds the electric circuit producing the magnetic field and sparkdischarge.

A further advantage of the present invention as compared with prior artis that, in the device of the invention, no large quantities of gas orhigh frequencies need to be used.

In a preferred embodiment of the present invention, the magnetic fieldand the spark discharge are produced by means of substantially the sameresonator circuit, consisting of a capacitor and a coil connected inseries. The load of the circuit, connected in parallel with the coil, isthe plasma forming space, which contains gas. As compared with theconventional parallel connection, the series connection has theadvantage that, when the load impedance falls as the gas is convertedinto plasma, the amplifier feeding the resonator circuit sees animpedance--the impedance caused by the capacitor--independent of theload impedance.

The frequency of the resonator circuit--a series connection of acapacitor and a coil--is automatically so selected that the circuitworks at the resonant frequency. In this case, the magnitudes ofcapacitance and reactance are equal, compensating each other. A suitablefrequency is in the RF range, typically in the range of 100 kHz-3 MHz.When frequencies higher than this are used, it is possible to use thenormal transmission path matching, i.e. a parallel connection of a coiland a capacitor, which is used in prior-art devices.

In a preferred case, the form and characteristics of the plasma beinggenerated are controlled by adjusting the power of the magnetic fieldand spark discharge and regulating the flow of the gas used.Furthermore, it is preferable to keep the power of the spark dischargeconstant and under control so that the discharge will not cause anyextra disturbance in the process of determining the presence ofelements.

The device of the invention for forming a plasma comprises a powersupply for supplying the power required for the formation of plasma, aplasma forming space, which is open in relation to its environment, anelectric circuit, which is electrically connected to the power supply toproduce a magnetic field and a spark discharge in the plasma formingspace, and a gas channel communicating with the plasma forming space forpassing a gas into the plasma forming space and out of it via its openpart. According to the invention, the electric circuit comprises aresonator circuit consisting of a series connection of a coil and acapacitor and arranged to connect the electric power needed for forminga plasma to the plasma forming space.

As for the advantages of the device of the invention, reference in madeto the advantages of the procedure of the invention.

In a preferred embodiment, the device comprises a first electrode, whichis electrically connected to a first potential of the electric circuit,and a second electrode, which is placed at a distance from the firstelectrode and electrically connected to a second potential in theelectric circuit, said first and second potentials being substantiallydifferent in magnitude. Further, the electrodes are so disposed that aspark discharge takes place in the plasma forming space with theselected values of the first and second potentials. In a preferredembodiment, one of the potentials is the earth potential of theamplifier feeding the electric circuit. The first and second electrodesare needed especially when treating gases that are difficult to convertinto plasma. Such gases include e.g. nitrogen. On the other hand, whentreating gases that are easier to convert into plasma, the secondelectrode is not necessarily needed at all. Such gases include e.g.argon. In this case, the second potential consists in the surroundingspace, and the spark discharge shoots from the tip of the firstelectrode out into space, e.g. through a coil placed in the direction ofthe tip. Resonance preferably prevails between the coil and thecapacitor, and the spark jet can be directed through a torque tube witha magnetic field on it.

In a preferred embodiment, the coil is disposed in the vicinity of theplasma forming space in such a way that the magnetic field generated bythe coil is perpendicular to the direction of the gas flow. In thiscase, the coil may be so disposed that the plasma forming space isinside the coil structure. On the other hand, the magnetic fieldproduced by the coil is also present outside the coil, so it is possibleto dispose the plasma forming space outside the coil. In practice,however, the tact is that the magnetic flux density is greatest insidethe cell. The coil preferably comprises a specified number of successivespiral discs with crossed windings. In such a solution, the winding isarranged in a spiral pattern on a round disc, starting near the centerof the disc.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF DRAWING

In the following, the invention is described by the aid of examples ofits embodiments by referring to the attached drawing, in which

FIG. 1 presents a diagram representing a device as provided by theinvention;

FIG. 2 presents a diagram representing a spiral disc forming part of thecoil of the device in FIG. 1;

FIG. 3a presents a conventional circuit for generating a magnetic field;

FIG. 3b presents the circuit used in the device of FIG. 1 for generatinga magnetic field and a spark discharge;

FIGS. 4-7 present simulation results for the circuits in FIG. 3a andFIG. 3b; and

FIG. 8 presents a diagram representing another device according to theinvention, resembling the device in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The device for generating a plasma as presented in FIG. 1 comprises apower supply 1, which preferably outputs a 200-V alternating voltage inthe frequency range of 100 kHz-3 MHz, and a plasma forming space 2 opento its environment, into which space a gas to be ionized is supplied.Furthermore,, the device comprises an electric circuit 3, whichaccording to the invention is a series connection of a coil and acapacitor and is electrically connected to the, power supply 1 togenerate a magnetic field and a spark discharge 14 in the plasma formingspace 2. As shown in FIG. 1, adjoined to the plasma forming space is awall 7 which also functions as a first electrode, being electricallyconnected to the earth potential of the power supply 1. Further, thedevice comprises a gas channel 4 communicating with the plasma formingspace 2 for passing gas into the plasma forming space and out of it viaits open part. The device presented in FIG. 1 has a second bar-likeelectrode 8 attached to the frame and preferably made of an electricallyconductive material. In FIG. 1, the plasma 12 being formed isrepresented by elliptic lines.

FIG. 2 presents a structure in which a conductor wire 13 is arranged ina spiral form on a disc-like body 11. The conductor wire 13 is woundalternately on either side of the disc 11. As the magnetic flux densityin the circuit used in the device of the invention is proportional tothe number of winding turns, the coil structure shown in FIG. 2 is veryadvantageous. Referring again to FIG. 1, the coil 5 comprises severalspiral discs as shown in FIG. 2, connected in series. The cooling ofsuch a coil structure is simple to implement and can be advantageouslyeffected by blowing air into the gaps between the discs.

Referring to FIGS. 3a and 3b and to the curves shown in FIGS. 4-7, theseries connection of the invention is compared with t:he conventionalparallel connection used for matching the transfer path and generating amagnetic field. The action of the circuits was simulated usingappropriate simulation software. The simulation results are presented inFIG. 4-7, in which the horizontal axis represents the frequency of thevoltage supplied by the power supply 5 and also the frequency of theresonator. In FIG. 4 and 5, the vertical axis represents the power, inFIG. 6 the current and in FIG. 7 the voltage. In addition, thesimulation program was given an external temperature value of 60° C.

The load impedance is represented in FIG. 3a and 3b by resistors R1 andR2, respectively. The load is connected in parallel with th,e coilproducing the magnetic field, affecting the current that flows throughthe coil. When the gas is transformed into plasma, the electricconductivity of the gas is clearly improved, thus reducing the loadimpedance in this case, the high-power amplifier in FIG. 3a sees thefall in the load impedance directly and tries to supply more and morecurrent into the load, so the circuit becomes unstable and difficult tocontrol. In FIG. 3b, the load impedance of the amplifier does notchange, because it has a constant value depending on capacitor C2.Therefore, the circuit remains stable and under control.

When the simulation results are examined, it can be seen from FIG. 4-7that there is a definite difference between the conventional circuit andthe circuit of the invention. FIG. 4 presents the power supplied by theamplifier into the resonator and the power fed into the coil asfunctions of frequency. As is clearly manifest from the figure, thehighest power both from the amplifier and across the coil is achieved atthe resonant frequency. FIG. 5 also graphically illustrates thedifference between the conventional circuit and the circuit of theinvention regarding the power transferred by the coil. FIG. 6 shows thecurrent flowing through the load resistances R1 and R2 (plasma) as afunction of frequency. From this, too, one can draw the conclusion thatthe resonator of the invention is more effective than the conventionalresonator.

In FIG. 7, the voltage across the coil is presented as a function offrequent y and compared with the amplifier output voltage. It can beseen from FIG. 7 that the voltage across the coil, about 4 kV, achievedby the procedure of the invention is clearly higher than the amplifieroutput voltage (200 VAC). By contrast, the voltage across the coilachieved using the conventional parallel connection, about 1.8 kV,remains below the amplifier output voltage (2 kV).

The device presented in FIG. 8 mainly corresponds to the device shown inFIG. 1. However, the device in FIG. 8 comprises only one electrode 8;the device has no separate electrode connected to the earth potential ofthe power supply 1. In the embodiment in FIG. 8, the plasma is formed inthe plasma forming space 2 and shot into space through the torque tubeformed by the coil 5, i.e. through the magnetic field generated by thecoil. The device of FIG. 8 is particularly applicable in conjunctionwith treating gases convertible into the plasma state, such as argon.

As a summary, the following can be stated. Plasma generated by means ofa spark and maintained by means of a spark and a magnetic fieldaccording to the invention becomes stabilized at the series resonancefrequency because the net effect of the spark diminishes as the voltagerises and vice versa, and when the power transferred via the magneticfield to the plasma increases, the voltage falls and the effect of themagnetic field diminishes. Moreover, amplifier noise and otherinterference voltages in the series circuit are attenuated according tothe proportion of the impedances.

The invention is not limited to the embodiment examples described above,but many variations are possible within the framework of the inventiveidea defined by the claims.

We claim:
 1. Procedure for forming a plasma, in which procedure amagnetic field is set up in a plasma forming space, a spark discharge isproduced in the plasma forming space and a gas flow is passed into themagnetic field in the plasma forming space, wherein, the plasma isformed in the plasma forming space by means of the spark discharge andmaintained by means of the magnetic field and spark discharge, andwherein the magnetic field and the spark discharge are produced by meansof substantially the same resonator circuit, comprising a seriesconnection of a capacitor and a coil.
 2. Procedure as defined in claim1, characterized in that the plasma is formed substantially within thecoil.
 3. Procedure as defined in claim 1, characterized in that theresonator circuit is supplied with an alternating electric current, thefrequency of which is selected automatically so that the resonatorcircuit works at the resonant frequency.
 4. Procedure as defined inclaim 3, characterized in that the plasma is controlled by adjusting thepower of the alternating current.
 5. Procedure as defined in claim 1,characterized in that the plasma is controlled by adjusting the volumeand/or rate of the gas flow.
 6. Procedure as defined in claim 1,characterized in that the spark discharge is produced in the plasmaforming space by means of an electrode placed in the gas flow andanother electrode placed in conjunction with the plasma forming space.7. Procedure as defined in claim 6, characterized in that the sparkdischarge is produced in the plasma forming space by means of theelectrode placed in the gas flow by directing the spark dischargethrough the magnetic field generated by the coil into the surroundingspace, which forms the other electrode.
 8. Procedure as defined in claim1, characterized in that the resonator circuit is supplied with analternating electric current and that the plasma is controlled byadjusting the power of the alternating current.
 9. Procedure as definedin claim 1, characterized in that the spark discharge is produced in theplasma forming space by means of an electrode placed in the gas flow bydirecting the spark discharge through the magnetic field generated bythe coil into the surrounding space, which forms another electrode. 10.Device for forming a plasma, comprisinga power supply (1) for supplyingthe power required for forming a plasma; a plasma forming space (2),which in open to the environment; an electric circuit (3), which iselectrically connected to the power supply to produce a magnetic fieldand a spark discharge in the plasma forming space; and a gas channel (4)communicating with the plasma forming space for passing a gas into theplasma forming space and out of it via its open part, and wherein theelectric circuit comprises a resonator circuit which comprises a seriesconnection of a coil (5) and a capacitor (6) and arranged to connect theelectric power required for forming a plasma to the plasma forming space(2) so that the magnetic field and the spark discharge are produced bymeans of substantially the said resonator circuit.
 11. Device as definedin claims 10, characterized in that the plasma forming space is disposedinside the coil (5).
 12. Device as defined in claim 10, characterized inthat the coil comprises a specified number of spaced coaxial discs (11)with spiral windings.
 13. Device as defined in claim 10, characterizedin that the device comprises a first electrode (7), which iselectrically connected to a first potential of the electric circuit, anda second electrode (8), which is placed at a distance from the firstelectrode and electrically connected to a second potential of theelectric circuit, said first and second potentials being substantiallydifferent in magnitude; and that the electrodes are so disposed that aspark discharge takes place in the plasma forming space (2) with theselected values of the first and second potentials.
 14. Device asdefined in claim 10, characterized in that the coil (5) is disposed inthe vicinity of the plasma forming space (2) in such a way that themagnetic field generated by the coil is perpendicular to the directionof the gas flow.